Project Summary/Abstract With improvements in systemic cancer treatment, the incidence of brain metastases (BM) is increasing but prognosis remains poor as 50% of patients die from their BM. Consequently, there is a critical need to better understand the biology of BM in humans in order to improve treatment for this challenging patient population. While radiation has been the traditional mainstay of treatment for BM, there have been an increasing number of clinical trials using chemotherapy and immunotherapy for BM to try to avoid the negative cognitive impact of radiation to the brain. In this proposal, we aim to study the response of patients with BM enrolled in a clinical trial of pembrolizumab, a PD-1 inhibitor. We will incorporate advanced MRI scans into the trial to better understand the biological response of BM to immunotherapy. One particular challenge with immunotherapy is interpreting radiographic responses so Aim 1 will use vascular MR imaging to determine if immune activation results in a different impact on tumor vascular structure and function than true tumor growth. Aim 2 will evaluate tumor heterogeneity since recent data suggests that the degree of tumor heterogeneity may influence response to immunotherapy. Aim 3 will correlate the MRI parameters with tumor tissue. The structure of the trial will allow us to look at baseline MRI parameters as well as longitudinal changes during therapy to shed light on the complex evolution of tumor biology. By looking directly at human BM tumor heterogeneity and response to immunotherapy, the proposed studies will provide greater biological insight into the behavior of these difficult- to-treat tumors and identify noninvasive, clinically applicable MRI parameters that can improve the interpretation of immunotherapy response and ultimately patient management.